1. Field of the Invention
The present invention relates to a method of protecting a silica-containing article used in the manufacture of an optical fiber, a method of inhibiting breaks during drawing of an optical fiber, and an intermediate product used in the manufacture of an optical fiber and protected against break-inducing particulates.
2. Description of the Related Art
An optical fiber is typically formed by drawing the optical fiber from a fiber preform heated to a high temperature. The fiber preform can be formed by a variety of processes. One such process, which is known as the outside vapor deposition process, is performed by applying silica soot to an alumina bait rod to establish a core profile, consolidating the core profile to create a glass core blank, and drawing the core blank to a smaller diameter to create a glass core cane. The core cane is then coated with soot, which is consolidated to create the fiber preform. Other processes, such as modified vapor deposition (MCVD) or plasma-activated chemical vapor deposition (PCVD), known generally as inside vapor deposition processes, are performed by depositing silica on the inside of a solid glass tube. The solid glass tube with the deposit is then collapsed to form a glass core blank, and glass is added to the outside of the core blank to form a fiber preform from which the optical fiber is drawn. Alternatively, the solid glass tube with the deposit can be collapsed to directly form a fiber preform. Still another process employed to make a preform for drawing optical fiber is the vapor axial deposition (VAD) process. The present invention has applicability in at least all of these various vapor deposition techniques.
As used herein, the term fiber preform shall refer to an article from which a fiber can be drawn without having to add more silica-containing glass. Core blank and core cane shall be used to refer to articles that include at least part of (but not necessarily all of) the optical core of the resultant fiber. A core cane is a core blank which has been consolidated and drawn into a smaller diameter, intermediate product. Thus, in some manufacturing operations, a core blank (or core cane) may be formed, after which additional core and/or clad glass material will be added to the core blank (or core cane) to form a fiber preform.
During drawing of an optical fiber from a fiber preform, the optical fiber often will break. The reduction of breaks during drawing of optical fiber is a clear goal in the industry, especially since customers now request lengths of optical fiber greater than fifty kilometers.
Fiber breaks are believed to be caused at least in part by inorganic foreign particulates (e.g., ZrO2) that deposit on glass surfaces of intermediate products, such as the fiber preform, core blank, core cane, and glass tube, produced during the formation of the optical fiber. These glass surfaces are reactive and can form irreversible bonds with the inorganic particulates. As shown schematically in FIG. 2, inorganic particulates 20 bond with active sites, such as OH groups, on a glass surface 10 and become part of the glass surface 10. Therefore, the particulates cannot be readily removed during standard cleaning before fiber draw. These particulates cause structural failure during fiber draw. For example, inorganic particulates on the glass surface of the fiber preform, core blank, core cane, or the glass tube, are believed to be a main cause of external fiber breaks, which occur during the draw process. Inorganic particulates on the glass surfaces of the core blank, core cane, and glass rube are believed to sometimes cause fiber internal breaks.
The inorganic particulates are present in the environment of the manufacturing plant. In addition to merely falling unaided onto the glass surfaces of the intermediate products, the particulates may be attracted to the glass surfaces by static charge. Ironically, a static charge often develops due to efforts to clean the glass surfaces.
Particulates can be removed from the glass surfaces of the intermediate products by using hydrofluoric acid as a cleaning agent. Hydrofluoric acid, however, changes the dimensions of the intermediate product because it etches the glass surface. Hydrofluoric acid is also expensive to use because it is toxic. Thus, hydrofluoric-acid cleaning is not a desirable technique for reducing fiber breaks.
It might be possible to reduce fiber breaks by manufacturing in a clean room so that there are almost no particulates to deposit on the glass surfaces of the intermediate products. This, however, would not be cost efficient.
As embodied and broadly described herein, the invention comprises a method of protecting a silica-containing article used in the manufacture of an optical fiber and thereby inhibiting breaks during drawing of an optical fiber. The method includes the steps of providing a silica-containing article used in the manufacture of an optical fiber, and applying a protective layer to the silica-containing article. The article could be for example, a core blank, a core cane, a fiber preform, a glass tube used in an inside vapor deposition process, a sleeve tube used to build up the glass exterior to the core glass, or any other silica-containing article. Preferably, the silica-containing article is a glass (as opposed to unconsolidated silica soot) when the protective layer is applied.
The invention also comprises an intermediate product used in the manufacture of an optical fiber and protected against break-inducing particulates. The intermediate product includes a silica-containing article, and a protective layer.
It is to be understood that the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the invention as claimed.